The invention relates generally to a method which employs nuclear magnetic resonance (NMR) imaging to determine properties of liquid-containing porous media, such as a rock core plug. More particularly and according to one aspect, the invention relates to determination of the liquid saturation of a porous media using NMR imaging. According to another aspect, the invention even more particularly relates to a method employing NMR imaging which determines the relationship between saturation and capillary pressure for a liquid-containing porous media. This relationship can be expressed in terms of a function which is sometimes called a capillary pressure curve (i.e. saturation versus capillary pressure).
The capillary pressure curve is a very important and useful tool in hydrocarbon reservoir engineering. Such a curve can be used in reservoir simulation to determine characteristics of a particular reservoir such as, for example, recoverable reserves or the likelihood of success for various enhanced oil recovery processes, such as water flooding.
Capillary pressure curves are most typically determined using the Hassler and Brunner method as described in "Measurement of Capillary Pressures in Small Core Samples", Trans. AIME, vol. 160, pp. 114-123 (1945). This method involves measuring the average saturation of liquid in a core plug at a plurality of different centrifuge speeds. Average saturation is determined from either the weight loss of the core plug after each centrifuge run, or from a measurement of the amount of liquid expelled by the core plug during each run. A capillary pressure curve is then determined utilizing the different average saturation and capillary pressure values corresponding to the various centrifuge speeds.
The Hassler and Brunner method has been widely and successfully practiced in the oil industry to determine capillary pressure curves, but has been found to suffer from several disadvantages. For example, the method requires a number (i.e. five to ten or more) of centrifuge runs which are time consuming (i.e. typically one to several hours per run) to carry out, and the method can give inaccurate results due to the fact that saturation at any given point (corresponding to a particular pressure) in the core is not known. Only average saturation is known. Therefore, approximations must be used which require a number of assumptions, including uniform displacement of the liquid, core homogeneity and 100% saturation at one end of the core, which are not necessarily valid. Furthermore, if average saturation is determined by the weight loss technique, which requires weighing, cleaning and resaturation of the core plug after each centrifuge run, the core plug is subjected to a great deal of handling which can cause loss of grains from the core plug during testing with a consequent unknown effect on core characteristics such as pore volume. This can further contribute to inaccuracy of the final results.